Many endpoints that serialize events don't include the transaction_id

[sandhose]

According to the spec, there are many endpoints that should include the transaction_id in the unsigned section of the event. Those endpoints are:

/sync
/rooms/{roomId}/state
/rooms/{roomId}/state/{eventType}/{stateKey}
/rooms/{roomId}/context/{eventId}
/rooms/{roomId}/messages
/rooms/{roomId}/relations/{eventId}
/rooms/{roomId}/relations/{eventId}/{relType}
/rooms/{roomId}/event/{eventId}
/rooms/{roomId}/threads
/search
/rooms/{roomId}/initialSync # Deprecated
/events # Deprecated

Synapse only does that on the /sync endpoint. I have a Complement test to highlight that issue using the /rooms/{roomId}/event/{eventId} endpoint.